Triacs and thyristors with a vertical structure are electronic power switches comprising a stack of at least four layers and/or semiconductor regions of alternated conductivity types. In such components, a first metallization or main electrode A1 rests on a main surface of the stack. A second metallization or main electrode A2 rests on the other main surface of the stack. A metallization or gate electrode G rests on the same surface of the stack as main electrode A1.
Generally, when a potential difference is present between main electrodes A1 and A2 of one of these components, the flowing of an electric current between main electrodes A1 and A2 is conditioned by the application of a gate current on the gate electrode. Once the current flow has been established between the main electrodes, these components keep on conducting the current until it passes under a threshold value, called the hold current.
FIG. 1 corresponds to FIG. 7 of United States Patent Application Publication No. 2015/0108537 (incorporated by reference). This drawing is a cross-section view of an example of a triac 1 having a vertical structure.
Triac 1 is formed from a lightly-doped N-type silicon substrate 3 (N−). The upper and lower surfaces of substrate 3 comprise P-type doped layers 5 and 7. Upper layer 5 contains a heavily-doped N-type region 9 (N+) and a heavily-doped N-type region 11 (N+). Lower layer 7 contains a heavily-doped N-type region 13 (N+) in an area substantially complementary to that occupied, in top view, by region 9. A main electrode A1 is arranged on the upper surface of substrate 3, astride N+ region 9 and a portion of P-type doped layer 5. A main electrode A2 is arranged on the lower surface of substrate 3, astride N+ region 13 and a portion of P-type doped layer 7. A gate electrode G is arranged on the upper surface of substrate 3, astride N+ region 11 and a portion of P-type doped layer 5.
When triac 1 is in the off state and a gate signal is applied to terminal G, a gate current IGK flows through P layer 5 between terminals G and A1, P layer 5 forming a resistor RGK between the terminals. If gate current IGK is greater, in absolute value, than the turn-on threshold of triac 1, voltage drop VGK between terminals G and A1 is sufficient to turn on triac 1, which switches from the off state to the on state.
A disadvantage of triac 1 is that, when its temperature increases, the resistivity of P layer 5, and thus the value of equivalent resistance RGK, increases. Thus, the flowing of a current IGK between terminals G and A1, even if this current is smaller than the normal turn-on current, may cause a high voltage drop in P layer 5, causing a parasitic turning-on of triac 1. Such a parasitic starting contributes to increasing the triac temperature up to a high value capable of causing a degradation, or even a destruction, of the triac. The same problem is posed with thyristors.
FIG. 2 corresponds to FIG. 3(a) of United States Patent Application Publication No. 2012/0250200 (incorporated by reference). This drawing is the electric circuit diagram of an example of protection of a triac 20 against overheating.
Triac 20 comprises main terminals A1 and A2, and a gate terminal G. A Shockley diode 30 is connected between terminal G and terminal A1 of triac 20. Shockley diode 30 is thermally linked to triac 20.
In operation, Shockley diode 30 is initially in the off state and triac 20 operates normally. When triac 20 is overheating, the temperature of the Shockley diode increases and its turn-on threshold decreases. Thus, when a gate signal is applied to terminal G, this signal is deviated through the Shockley diode. As a result, no current flows through resistor RGK shorted by the Shockley diode, and triac 20 remains in the off state. This enables to avoid for the triac temperature to continue increasing, thus preventing its degradation or its destruction.
The protection described in relation with FIG. 2 is relatively difficult to implement and to adjust and is further relatively bulky.
It would thus be desirable to have triacs and thyristors comprising a protection against overheating overcoming at least certain disadvantages of existing protections.